HortJ

In contrast to fluorescent lamps and high-power sodium lamps, the use of light-emitting diode (LED) lamps enables the control of not only photosynthetic photon flux density (PPFD) at the plant level, but also the relative spectral photon flux density distribution (RSPD) of light because of the variety, even at different times of day, of producible light emitted by LEDs of different types. Effects of the spectral photon flux density on plant growth and morphology have been investigated using several types of LEDs and plant species. However, few studies on lighting methods with time-varying PPFD or RSPD have been published to date. In this paper, we summarize the effects of time-varying PPFD on the net photosynthetic rate (P_n) and those of time-varying RSPD on plant growth and morphology. Detailed modeling studies have been conducted on the reactions of the photosynthetic pathway under time-varying PPFD at a cycle of milliseconds to seconds. The results of these modeling studies and actual measurements of P_n under pulsed light clearly indicate that pulsed light is not advantageous to improve P_n. Although the integrated PPFD of blue and red light was unchanged, the growth of leaf lettuce was promoted by asynchronous irradiation with blue light and red light compared with growth under simultaneous irradiation. We think that blue-light monochromatic irradiation promotes leaf elongation through leaf expansion as a primary factor in the enhancement of plant growth. In addition, changes in leaf photosynthetic capacity caused by blue-light monochromatic irradiation may be involved in plant growth promotion. An increasing number of studies have investigated the effects of time-varying RSPD on plants. However, the mechanisms underlying these effects remain to be elucidated.

Recently, the number of patients with chronic kidney disease has increased rapidly and kidneys with loss of the K-defecating function have been observed. Thus, providing vegetables with low potassium is an urgent unmet need. In this study, two cultivation methods were used to cultivate lettuce (Lactuca sativa L.) with low K concentrations. One method, dubbed LKEC, was based on electrical conductance management and the K supply was stopped at the end of cultivation. The other method, dubbed LKQM, was based on nutrient quantitative management, and the nutrients required for low-K lettuce were quantitatively supplied. Meanwhile, control lettuce with a normal K concentration, known as CK, were cultivated with electrical conductance management. Compared with CK, both low K treatments reduced the yield by nearly 20% without any visual deficiency symptoms. There was no significant difference between LKEC and LKQM in terms of plant growth. LKQM-treated lettuce contained lower Na and required less fertilizer than LKEC lettuce. Moreover, these plants adapted to K deficiency stress by absorbing more cations to maintain osmotic pressure. N declined with decreasing K. This suggested that the quantitative management method in low-potassium lettuce production reduced the potassium content in the lettuce plants to the same level as the EC management method, and significantly reduced the sodium content compared to EC management.

Low-potassium crops are required for patients with kidney disease, and research on the production of low-potassium vegetables using hydroponics has been conducted. However, there are few studies on low-potassium fruit trees because the soil is generally cultivated. This study focused on blueberries that can be cultivated in a pot, by examining the production of low-potassium blueberry fruits cultured with fertigation in a greenhouse. In a pot culture using peat moss medium, the potassium levels were restricted from the flowering stage and from the fruit coloring stage, causing a decrease in the potassium content of the fruits by 53 and 35%, respectively, when compared with the control. A urethane sponge-based medium with free nutrient leaching was then evaluated to determine whether the potassium content of fruits decreased with short-term potassium restriction. The results showed a reduction in potassium content of 48% when potassium was restricted in the fruit coloring period. In addition, potassium was restricted for five months to determine whether long-term potassium restriction could further reduce the potassium content of fruits. The fruit potassium content did not differ between the second and fifth months after the potassium restriction, although symptoms of potassium deficiency appeared in mature leaves. From these results, it was suggested that the pot culture with fertigation was effective in producing low-potassium blueberry fruits, and the fruit potassium content can be halved by short-term potassium restriction using the urethane sponge-based medium. However, long-term potassium restriction was not effective in producing low-potassium blueberry fruits due to the appearance of symptoms of potassium deficiency.

Here, we describe the current status of energy-saving techniques in greenhouse heating and countermeasures for high summer temperatures, and then introduce an outline and the performance of a newly developed New Root-zone Environmental Control System (N.RECS) and the effects of this system on flower production. Temperature control techniques used in protected horticulture include those that control the air temperature of the entire greenhouse space and those that control the local temperature of the plants. The running costs of the air temperature control techniques are expensive and the cooling capacity in the summer are inadequate for actual cultivation. Local temperature control techniques have relatively low running costs and can adequately cool plants during high temperature periods, but they are crop-specific and not versatile. Therefore, we developed a N.RECS that can heat and cool the root-zone of various plants with low running costs. N.RECS was produced by combining aluminum heat exchange panels, insulated pot trays and an air-source heat pump cold/hot water supply system. By using N.RECS, the root-zone temperature was maintained at 25°C when the air temperature was about 5°C during the winter and cooled to below 23°C when the air temperature was about 35°C during the summer. Root-zone heating of six potted flowers and cut flowers in the winter promoted growth and flowering and achieved energy savings of about 30% compared to conventional heating.

When the two flower species, which are susceptible to high temperature, were grown under high temperature conditions, about half of the plants died, but root-zone cooling allowed all the plants to grow healthily. Therefore, it is concluded that N.RECS is a practical system that allows for both energy-saving cultivation in the winter and cooling cultivation in the summer. In addition, root-zone temperature is an environmental control parameter that should be controlled in future agricultural production; by applying root-zone temperature control as a standard technology for agricultural production, the growth of plants can be controlled more flexibly and more energy can be saved.

To improve the yield of sweet peppers in Japan, we investigated the relationships between plant growth characteristics and fruit yield, with a particular focus on dry matter production. Fruit fresh weight (i.e., fruit yield) was highly correlated with fruit dry weight. Aboveground dry matter production was correlated with both light use efficiency and intercepted photosynthetically active radiation. Although the number of fruits was correlated with the fruit set ratio, no correlation was found between the number of fruit and fraction of dry matter distributed to fruit. The correlation between the fraction of dry matter partitioned to fruit and fruit dry weight varied by plant stage, potentially due to the yield fluctuation called “flush”. Together, these findings suggest that to improve the yield of sweet peppers in Japan, we should focus first on improving light use efficiency.

The effects of End-of-Day (EOD)-cooling on floral differentiation and cut flower characteristics in carnation were verified using both short-term model-based experiments in a growth chamber and long-term demonstrative experiments in a greenhouse. In the model experiment, flower buds were undifferentiated until 43 days after pinching in both control and End-of-Night-cooled plants. In contrast, in the EOD-cooling treatment, every set reached the reproductive phase by 22 days after treatment. At the same node position, a three-stage advancement in floral differentiation was observed after EOD-cooling, but not after the other two treatments. This suggested that EOD-cooling promoted both floral differentiation and development in carnation. For the demonstrative experiment, significant differences between EOD-cooled and control plants varied each year in days to flowering, node order of flowering, and stem weeping angle. Because environmental factors could affect variation, correlations between the diurnal temperature range and the difference in each survey component were examined using regression analyses. Moderate (days to flowering) and a strong (node order of flowering) correlations were found with the diurnal temperature range. Thus, the EOD-cooling treatment was advantageous in suppressing delayed flowering over a large diurnal temperature range during floral differentiation. The difference in stem weeping angle showed a strong correlation with the difference in minimum temperature during the stem elongation stage. Improving the stem quality by EOD-cooling tended to be effective under high minimum-temperature conditions.

The growth and yield of tomatoes in greenhouses covered by heat insulation film were investigated in Ishigaki, a subtropical area of Japan with high temperature and high solar radiation. High-density planting of tomatoes was carried out in June, July, August, October, and November, with a low node-order pinching system. The plants were grown in two greenhouses, one covered by a thermal barrier film (TBF) for heat insulation and the other covered by a polyolefin film (PO). Under TBF, the maximum temperature was up to 3.6°C lower than that under PO, and the mean temperature was 0.4°C lower. The leaf area index (LAI) of the June planting was 3.0 under TBF and 3.4 under PO, significantly lower than the LAIs of the other plantings, which ranged from 3.9 to 5.4. The specific leaf area (SLA) varied with the cultivation season, with that under TBF and PO being highest in November plantings grown in winter. SLA under TBF was higher than that under PO in the same cultivation seasons. The amount of total aboveground dry matter was significantly lower in the June planting under TBF and showed no difference in other plantings. Distribution to fruit was higher under TBF than under PO, especially in summer cultivation. The yields under TBF were significantly higher than those under PO in the July planting, and the yields under PO were significantly higher in the November planting. Under TBF, the rate of fruit cracking was reduced in the June, July, and August plantings. Under TBF and PO, plantings in October and November produced puffy fruits. The light use efficiency under TBF was higher than that under PO in the same cultivation seasons, apart from the June planting. Tomato cultivation under TBF reduced the damage from intense solar radiation during summer. TBF enabled penetration of sufficient solar radiation to distribute photosynthates to each part of the plant because of a limited sink and abundant solar radiation in a subtropical summer. These results indicate that TBF and sink-limited cultivation are effective systems for use in the high temperature and high solar radiation environments of tropical and subtropical regions.

Sweet basil (Ocimum basilicum L.), one of the most widely consumed herbs globally, is used in raw or processed food, and for aromatic essential oils. We investigated the effects of photosynthetic photon flux density (PPFD; 150, 225, and 300 μmol·m⁻²·s⁻¹, herein, referred to as P150, P225, and P300) and red to blue light ratios (R/B ratio) (R:B = 1:4, 1:1, and 4:1, herein, referred to as R/B 0.25, 1.0, and 4.0) with a 16 h light period on the leaf shape and concentrations of functional and aromatic compounds in basil. Total leaf dry weight and leaf mass per area increased with increasing PPFD and R/B ratio. Total leaf area tended to increase with increasing R/B ratio at the same PPFD. Although the highest growth was noted when R/B was 4.0 at P300, the leaves showed ruggedness and curling. β-Carotene concentration based on the leaf dry weight and leaf area at the fourth node increased with decreasing R/B ratio, regardless of PPFD. Concentrations of aromatic compounds (eugenol and linalool) based on dry weight were significantly higher at P150 than at other PPFDs and in treatments with greater amounts of red light. These results suggest that basil growth, appearance, and functional and aromatic compound concentrations can be adjusted as needed by manipulating the PPFD and R/B ratio, although R/B 4.0 at P300 caused malformed leaves.

We applied the New Root-zone Environmental Control System (N.RECS), which comprises floor heating panels with chilled or heated water flow and heat-insulating pot trays, to cool the root zone of pot-grown strawberry (Fragaria × ananassa Duch.) nurseries in late summer, and investigated the effects on the subsequent flowering and yield in a super-forcing culture. Cooling at around 20°C with short-day (8-h daylength) treatment for 22 days accelerated and stabilized the flower bud formation of two June-bearing cultivars, ‘Nyoho’ and ‘Tochiotome’. The temperature at the soil surface, where the crown is located, was around 23°C on average during root-zone cooling, possibly acting as a cold stimulus on the shoot apical meristem, which initiated flowering. Root-zone cooling did not affect the development of new leaves during treatment. Combined root-zone cooling with short-day treatment promoted anthesis of the first inflorescence to the same degree as conventional night cooling, but neither did so alone. Weight-based early marketable yield before December was doubled but the total yield during October to March was not increased. These data suggest that root-zone cooling with short-day treatment of June-bearing strawberries may be applicable for super-forcing culture harvested from October to mid-November.

Many strawberry cultivars are developed in Japan each year; however, the characterization of each cultivar, including yield, is insufficient. As they may affect strawberry yield, leaf photosynthesis characteristics were determined for seven Japanese strawberry cultivars (‘Akihime’, ‘Benihoppe’, ‘Ibarakiss’, ‘Oi C berry’, ‘Sachinoka’, ‘Suzuakane’, and ‘Yotsuboshi’) and evaluated with reference to a C₃ photosynthesis model. Plants were grown hydroponically with a substrate-filled elevated-bed system in a glass greenhouse. Leaf gas-exchange rates were assessed at different photosynthetic photon fluxes and CO₂ partial pressures using full-expanded leaves of plants 130–150 days after transplanting (DAT). We found that the net photosynthetic rate (Pn) was higher in ‘Yotsuboshi’ and ‘Sachinoka’ than in other cultivars both under light-limited and light-saturated conditions. The difference in Pn was correlated with stomatal conductance (gs), irrespective of light conditions. We also measured leaf Pn under both CO₂-limited and CO₂-saturated conditions; however, there were only a slight differences in either capacity of ribulose-1,5-bisphosphate carboxylation or regeneration among cultivars. Our results suggest that a difference in gs contributed to variations in leaf Pn among Japanese strawberry cultivars.

Dwarf cherry tomatoes ‘Red Robin’ and ‘Tiny Tim Red’ were cultivated in two sections. The two sections were set as (1) section irradiated only by fluorescent lamps (main wavelengths 433, 543, and 610 nm; 230 μmol·m⁻²·s⁻¹ of photosynthetic photon flux density) and non-treated with UV-A irradiation (hereafter, non-UVA), and (2) section irradiated with fluorescent lamps and treated with UV-A irradiation (maximum peak emission: 352 nm; 15.1 W·m⁻² ultraviolet radiation intensity; hereafter, t-UVA). The fruit weight per plant and the number of fruits per plant were not significantly affected by cultivar or section. Regarding fruit quality, the fruit cracking rate was lower in the t-UVA than in non-UVA section. There were no anatomical or histochemical differences in fruit structure or distribution of pigments such as lycopene or β-carotene, but the shape of the pigment in the epidermal cells was needle-shaped in the non-UVA and unclearly-shaped in the t-UVA, so the pigment shape was different between the two sections. In addition, the number of layers of the hypodermis in the exocarp that accumulated the pigment was smaller in the t-UVA than in non-UVA section. Fruit components in t-UVA showed increased total soluble solids (TSS), titratable acidity (TA), and ascorbic acid content, but a decreased carotenoid content. Therefore, it was shown that UV-A irradiation had a positive effect on the TSS, TA and ascorbic acid content and had a negative effect on the size of the fruit per fruit and the carotenoid content. UV irradiation did not affect the yield per plant. Comparing ‘Red Robin’ and ‘Tiny Tim Red’, there was no significant difference in yield, but the fruit size of ‘Red Robin’ was larger, while and the fruit component of ‘Tiny Tim Red’ was higher than that of ‘Red Robin’. In conclusion, when cultivating tomato plants in a plant factory, further improvements in fruit size and composition are expected by appropriately adjusting the time and intensity of UV irradiation for each cultivar.

The flower seed industry is facing a variety of issues related to ongoing climate change. The closed plant production system (CPPS) may be a solution to these issues as the environment inside the system can be fully controlled to allow seed production from many plant species. In this study, the influence of an artificial light condition, one of the key factors influencing plant growth and development in CPPS, on gloxinia (Sinningia speciosa) seed production was investigated. Three types of lights: warm white (3200 K), daylight (6500 K), and red and blue lights (RB) at a photosynthetic photon flux density of either 150 or 200 μmol·m⁻²·s⁻¹ were applied to gloxinia plants. The highest vegetative growth was found in plants grown under 3200 K at 200 μmol·m⁻²·s⁻¹ light but this did not correlate with seed yield. Plants grown under RB light exhibited the most compact canopy. Day to anthesis, flower diameter and percentage of pod set were not significantly different among the light treatments. Gloxinia plants grown under RB light at 150 μmol·m⁻²·s⁻¹ had the highest seed yield, which is attributed to higher pollen elongation and flower number. The effect of light quality on seed yield is strongly dependent on light intensity. Moreover, treatment with RB light resulted in longer pollen grains and seeds than the other light quality treatments. Gloxinia seeds from all light treatment exhibited more than 80% germination and similar seed vigor. The results from this study suggest that CPPS, with suitable light conditions, may be used for commercial gloxinia seed production.